Vacuum cooling is a well known and proven cooling method within the fresh food produce industry. The method is widely used for cooling fresh produce from temperatures of up to about 30° C. (85° F.) to storage temperature of about 2° C. (35° F.) within very short time. Because the cooling method relies on latent heat of vaporization of water at low pressure in order to effect the cooling of the produce, the method is often combined with the addition of water. The refrigeration system may comprise a number of refrigeration coils placed inside the vacuum chamber which coils condense the vapor back into water, thereby making the process to work more efficiently and preventing water vapor to be carried over to the vacuum pump system. Until the present invention, vacuum cooling systems typically use a fixed speed two stage vacuum pump, e.g. of the rotary sliding vane type, for lowering the pressure and maintaining the lowered pressure inside the vacuum chamber, which is alternatively referred to in the industry as the “bounce” point. Often the two stage vacuum pumps used are somewhat undersized for the first pressure lowering step, but oversized for the second pressure maintaining step. Because the two stages of the pumps are directly coupled together, the high pressure stage is still being rotated during the pressure lowering step even though it is only the low pressure stage of the pumps that is functioning. This entails higher energy consumption than theoretical necessary, a fact which is supported by practical experience showing that less than 10% of the vacuum pump volumetric capacity is used during the pressure maintaining step, but the entire vacuum pump continues to run at full speed.
The object of the present invention is to provide a method as well as an apparatus for vacuum cooling of produce through moisture removal, by which the above disadvantage is eliminated or at least significantly reduced.